The production of oil and natural gas from an underground well (subterranean formation) can be stimulated by a technique called hydraulic fracturing, in which a viscous fluid composition (fracturing fluid) containing a suspended proppant (e.g., sand, bauxite) is introduced into an oil or gas well via a conduit, such as tubing or casing, at a flow rate and a pressure which create, reopen and/or extend a fracture into the oil- or gas-containing formation. The proppant is carried into the fracture by the fluid composition and prevents closure of the formation after pressure is released. Leak-off of the fluid composition into the formation is limited by the fluid viscosity of the composition. Fluid viscosity also permits suspension of the proppant in the composition during the fracturing operation. Cross-linking agents, such as borates, titanates or zirconates are usually incorporated into the fluid composition to control viscosity.
Typically, less than one third of available oil is extracted from a well after it has been fractured before production rates decrease to a point at which recovery becomes uneconomical. Enhanced recovery of oil from such subterranean formations frequently involves attempting to displace the remaining crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) provide a bank of oil of substantial depth being driven into a producing well; however, in practice this is frequently not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling frequently occurs, so that the driving fluid flows preferentially through permeable zones depleted of oil (so-called “thief zones”) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable.
Difficulties in oil recovery due to thief zones may be corrected by injecting an aqueous solution of an organic polymer and a cross-linking agent into a subterranean formation under conditions where the polymer will be cross-linked to produce a gel, thus reducing permeability of the subterranean formation to driving fluid (gas, water, etc.). Polysaccharide- or partially hydrolyzed polyacrylamide-based fluids cross-linked with certain aluminum, titanium, zirconium, and boron based compounds are used in these enhanced oil recovery applications.
Cross-linked fluids or gels, whether for fracturing a subterranean formation or for reducing permeability of zones in subterranean formation, are now being used in wells under a variety of temperature and pH conditions, where rates of cross-linking with known cross-linking compositions may be unacceptable.
U.S. Pat. No. 4,883,605 discloses a water-soluble zirconium chelate formed from a tetraalkyl zirconate and hydroxyethyl-tris-(2-hydroxypropyl)ethylenediamine, and the use of the chelate as a cross-linking agent in hydraulic fracturing fluids and in gels that are used for selectively plugging permeable zones in subterranean formations or for plugging subterranean leaks.
The products of U.S. Pat. No. 4,883,605 are excellent cross-linkers for use in hotter, deeper oil well applications, where a significant delay in rate of cross-linking is required. For moderate temperature formations, however, these products cross-link too slowly (>10 minutes), causing a “sand out” to occur, which is the result of sand depositing at the bottom of the wellbore due to lack of viscosity development before the gel reaches the fracture zone.
A similar chelate, formed from zirconium tetrachloride and tetra-(2-hydroxypropyl)ethylenediamine and neutralized with sodium hydroxide is disclosed in U.S. Pat. No. 2,824,115, Example IV. This compound has been found to form a slurry in water and be unsuitable for use in these applications. In addition, the presence of sodium chloride in this composition would most likely cause syneresis of the cross-linking gels, rendering them less effective.
Commercially available zirconate cross-linkers, such as tetra-triethanolamine zirconate cross-link too fast, causing a significant loss in viscosity due to shear degradation, which can also result in a sand out.
Other zirconium complexes of triethanolamine, such as those disclosed in U.S. Pat. Nos. 4,578,488, 4,683,068, and 4,686,052 can be used as cross-linking agents, however, these complexes also do not cross-link at a desirable rate, especially in the hotter, deeper wells, causing a similar loss in viscosity due to shear degradation.
There is a need for compositions which cross-link at a rate intermediate between zirconium complexes of hydroxyethyl-tris-(2-hydroxypropyl)ethylenediamine and triethanolamine zirconates which can be used successfully in moderate temperature (250-350° F., 121-177° C.) formations.